Given the Polynesian history that we have, I would assume leapfrogging shoreline colonization, moving inland, and the formation of organized communities moving towards City states and then more formal states. It's too big for any single state to really predominate in the time available, and I'd bet the interior would be largely thinly populated. I'd say we'd see something similar to India or Indonesia, lots of states and principalities.
My Empire of Mu timeline deals with a similar Polynesian civilisation, but its geography gives it a head start of up to 1500 to 2000 years, which allows for the unification of the continent well before Europeans come along.
How big and how organized the Hawaiin polities are will depend a lot on the amount of contact and cultural diffusion with Southeast Asia. I'm willing to bet that there will be some contact, and likely some transmission of things like metallurgy, plants, animals, writing, etc.
Likely discoverers and explorers would be the Dutch, British and French in that order, through successive expeditions. Most likely the British would be the ones to define all the shorelines. But the Dutch are likely to find it first.
The Spanish or Portugese might encounter the most westerly outlying Islands, but unless there's real commercial potential, aren't likely to explore pointlessly.
As for colonization, a lot depends on circumstance. The British hold over India was an incremental and peacemeal thing. The same thing with the Dutch in Indonesia. The French were involved with Asia for centuries before they started taking over Indochina in the 19th century.
Given a non-unified polyglot of Post-Polynesian states, we might see different Colonial powers moving into Hawaii at different times.
Comparing it to Southeast Asia, we might see the easterly Islands nearest the Phillipines and Indonesia coming under the influence of Portugal. The Portugese are displaced by the Dutch a century later. The Dutch in turn find their position eroded by the English and French, all dealing with nominally independent states and city states.
In the 19th century, there's a European scramble for colonies. Russia gets involved in the north. Portugal and the Netherlands hold onto relic slices. France, Britain, Germany, Japan and the United States control territories directly. There's a struggle for the thinly populated Interior. A few of the more advanced states maintain their independence.
-How would this affect major wars, like the Pacific War in WW2.
It would undoubtedly be a theatre of war.
Not without some sort of unification, and this is unlikely given the timeline.